The present disclosure relates to a controller and a control method for an internal combustion engine that is provided with an ignition coil.
To date, as the ignition device of the internal combustion engine, there has been known an ignition system in which the voltage stepped up by the ignition coil is supplied to the ignition plug, the spark discharge (here, it means a dielectric breakdown and a subsequent formation of discharge plasma) is generated between the gap of the ignition plug disposed in the combustion chamber of the internal combustion engine, and the spark ignition is performed to the fuel-air mixture in the combustion chamber by energy which the spark discharge supplies.
In recent years, the demand to the ignition system becomes highly functional for downsizing by supercharging, high compression ratio, and high dilution combustion which are the trends aiming to improve the fuel efficiency of the internal combustion engine. That is, in the internal combustion engine downsized by supercharging, or the internal combustion engine of high compression ratio, there is the trend that the internal cylinder pressure at the time of spark ignition becomes high significantly, as compared with the conventional internal combustion engine; consequently, because breakdown voltage also becomes high, output energy increase of the ignition coil is required, and high withstand voltage performance of the ignition coil and the ignition plug is also required. High dilution combustion is high EGR combustion and high lean burn combustion. Such fuel-air mixture generally has a narrow stable combustion region. In order to burn this stably, it is known that it is effective to increase the output energy of the ignition coil, to extend the discharge period, to strengthen the in-cylinder flow, and the like.
By the way, when performing spark ignition in the internal combustion engine which can generate a strong in-cylinder flow using the ignition coil which increased output energy as described above, it is known that the phenomena that the discharge plasma which occurs between the gap of the ignition plug is flowed and extends long by the in-cylinder flow will occur. By being flowed and extending of the discharge plasma in this way, rather, the fuel-air mixture around the discharge plasma is activated, and the influence of cooling by the electrode also decreases because the discharge plasma departs from the ignition plug; therefore, it is known that even in high dilution combustion, it is effective in stabilization of combustion. This phenomenon is described in JP 2008-88947 A, JP 4978737 B, and JP 2015-200257 A, for example.
In the technology disclosed in JP 2008-88947 A, by interrupting discharge when the flow of discharge sparks is observed based on the ignition current value, and suppressing the difference between the case where the discharge was flowed and the case where the discharge was not flowed, the output fluctuation between cycles is suppressed. In the technology disclosed in JP 4978737 B, discharge path length is calculated based on discharge voltage, and the length of discharge is controlled by the electromagnet provided in the ignition plug. In the method disclosed in JP 2015-200257 A, an air flow speed in the combustion chamber is estimated, based on a change in slope of the secondary current accompanying a change in the secondary voltage.